Low band gap polymers for organic photovoltaics

Low band gap polymer materials and their application in organic photovoltaics (OPV) are reviewed. We detail the synthetic approaches to low band gap polymer materials starting from the early methodologies employing quinoid homopolymer structures to the current state of the art that relies on alternating copolymers of donor and acceptor groups where strategies for band gap design are possible. Current challenges for OPV such as chemical stability and energy level alignment are discussed. We finally provide a compilation of the most studied classes of low band gap materials and the results obtained in photovoltaic applications and give a tabular overview of rarely applied materials.     

Optimum design of ordered bulk heterojunction organic photovoltaics

Organic photovoltaics (OPVs) have recently received increasing attention as alternatives to inorganic solar cells because of their light weight, compatibility with flexible electronics, and low production cost. In this work, an ideal device structure for ordered bulk heterojunction (OBHJ) organic photovoltaics is proposed. Also suggested is a parameter related to the power conversion efficiency (PCE) of the devices. Such a parameter could serve as a rule of thumb for researchers. For cases in which it is difficult to reduce the pillar size and spacing, the proposed parameter dictates that an increase in the active layer thickness could be one way to increase the PCE of an OBHJ. To generate maximum PCE, (1) the pillar size and spacing must be two to three times less than the exciton diffusion length and (2) the thickness of the active layer has to be greater than half of the photon mean free path, which is the inverse of the absorption coefficient.     

Metal oxide applications in organic-based photovoltaics

Abstract

Organic-based photovoltaics (PV) have attracted increasing attention in recent years and efficiencies exceeding 8% have recently been confirmed. These low cost, lightweight and mechanically flexible devices offer unique advantages and opportunities currently unavailable with crystalline silicon technology. Progress in the field of organic PV has been achieved in part due to the incorporation of transition metal oxides. These offer a wide range of optical and electronic properties, making them applicable in organic-based PV in many capacities. Transparent electrodes can be made from doped metal oxides. The high intrinsic charge carrier mobility of many undoped metal oxides makes them attractive as active materials and charge collectors. Metal oxides can increase the charge selectivity of the electrodes due to the energetic positioning of their valence and conduction bands. Thin films of these materials can manipulate the light distribution inside of organic devices, allowing for improved light harvesting. Metal oxides are stable and can be processed at low temperatures. Consequently, they have been demonstrated as suitable intermediate layer materials in tandem cells. Finally, oxygen-deficient metal oxides can improve the stability of the oxygen- sensitive organic semiconductors. The present work reviews the various applications of metal oxide layers in organic PV devices and summarises the challenges associated with organic/oxide interfaces.     

Flexible Ag electrode for use in organic photovoltaics

High efficiency organic photovoltaic cells discussed in literature are normally restricted to devices fabricated on glass substrates. This is a consequence of the extreme brittleness and inflexibility of the commonly used transparent conductive oxide electrode, indium tin oxide (ITO). This shortcoming of ITO along with other concerns such as increasing scarcity of indium, migration of indium to organic layer, etc. makes it imperative to move away from ITO. Here we demonstrate a highly flexible Ag electrode that possesses low sheet resistances even in ultra-thin layers. It retains its conductivity under severe bending stresses where ITO fails completely. A P3HT:PCBM blend organic solar cell fabricated on this highly flexible electrode gives an efficiency of 2.3%.     

Battery effects in organic photovoltaics based on polybithiophene

Homopolymer photovoltaic devices based on thin films of polybithiophene, prepared by direct electrodeposition onto transparent fluorine-doped tin oxide electrodes followed by evaporation of an aluminium electrode to complete the device, were reported by Leguenza et al. [J. Solid State Electrochem. 11 (2007) 577.] to exhibit very high open-circuit voltages (V_oc) of up to 2 V at a very low light intensity of 6.6 W m⁻². In this letter, we report our attempts to reproduce the results. We achieved V_oc's0.8 V under a light intensity of 23 W m⁻². We also observed an unexpected V_oc in the dark suggesting that the high voltages previously reported might be attributed to the polarization of the polybithiophene layer, which lead to a battery effect in the dark and therefore not entirely a photovoltaic effect. We conclude that more work is needed before this observation can conceivably be applied in organic photovoltaics. Most notably, the application of this polarization effect depends on the demonstration of the high V_oc under high luminous intensities (i.e. >200 W m⁻²) and at high current densities (>1 mA cm⁻²) which has not been achieved yet.     

Low band gap poly-thienopyrazines for solar cells—Introducing the 11-thia-9,13-diaza-cyclopenta[b]triphenylenes

The chemistry of the thienopyrazines has been explored with the aim of producing new low band gap polymers. 5,7-Di-(thiophen-2-yl)-thieno[3,4-b]pyrazines substituted in the pyrazine ring with alkyl groups, aryl groups and fused aromatic rings have been prepared and characterized. The electronic spectra show a great variation in the longest wavelength absorption band as a consequence of this substitution. A special case is the 11-thia-9,13-diaza-cyclopenta[b]triphenylene prepared by condensation of 3′,4′-diamino-[2,2′,5′,2″]terthiophene with phenanthrene-9,10-quinone. Alkyl substitution of the most promising monomers were carried out using the Kumada coupling and these were copolymerized with either 2,5-bis(trimethylstannyl)thiophene or 3-(3,7,11-trimethyl-dodecyl)-2,5-bis-trimethylstannyl-thiophene to form six new low band gap polymers: RISO-GREEN 1–3 and RISO-BROWN 1–3. The band gaps of these polymers were estimated from the UV–visible absorption spectra and found to be ca. 1.3 eV. Preliminary results from photovoltaic device fabrication with mixtures of the six polymers with either [60]PCBM or [70]PCBM gave modest efficiencies of max 0.2% with open circuit voltages V_oc of 0.3 V and short circuit currents J_sc (1000 Wm⁻² AM1.5) in the range of 2 mA cm⁻².     

Organic and inorganic solar cells parameters evaluation from single I–V plot

This paper presents a new method to determine the five solar cell parameters of the single diode lumped circuit model. These parameters are usually the saturation current, the series resistance, the ideality factor, the shunt conductance and the photocurrent. This method is based on the measured current–voltage data. The method has been successfully applied to a commercial silicon solar cell, a module and an organic solar cell.     

Lifetimes of organic photovoltaics: photochemistry, atmosphere effects and barrier layers in ITO-MEHPPV:PCBM-aluminium devices

Large area polymer photovoltaic cells based on poly[(2-methoxy-5-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) were prepared. The lifetimes of the photovoltaic cells were studied in terms of the atmosphere, handling, electrode treatment, mode of preparation and barrier layers by recording the short circuit current as a function of time. Two exponential fits to the decay curves allowed for the extraction of the time constants for different degradation processes. For the periods of time studied here (24–300 h), the decay curves could be fitted with two exponential functions. Common to the preparations were that the first half-life remained short and was independent of the presence of oxygen. When fullerenes were employed by sublimation of a layer of C₆₀ or as the soluble PCBM, the first half-life was an order of magnitude longer and depended on the presence of oxygen. By employing different barrier layers, we found the first half-life to be linked to the aluminium polymer interface and ascribe it to a photochemical reaction between the organic material and the reactive aluminium at the interface. The second and longer half-life was found to depend on the presence of oxygen. We also discuss our findings of the short lifetimes for organic photovoltaics under AM1.5 illumination in the context of future applications.     

Influence of the hole-transport layer on the initial behavior and lifetime of inverted organic photovoltaics

The inverted organic photovoltaic (OPV) device architecture represents an important advancement due to the relative environmental stability of the electron transport layer (ETL) and hole-collecting contact. We investigated the initial and long-term behavior of inverted devices to identify changes taking place at the Ag hole-collecting contact. We show that efficient hole collection can be obtained after modifying the Ag contact by thermal annealing, long-term exposure to ambient atmosphere, or employing a high work function organic hole-transport layer (HTL). We find that whether or not the device employs an organic HTL, degradation of the photocurrent initially follows a simple exponential decay. After prolonged illumination (>500 h), devices with an organic HTL fail catastrophically due to a precipitous drop in photocurrent. Based on evidence for pinhole-induced degradation observed in photocurrent maps, we propose a nucleation and island growth mechanism and a model for the photocurrent behavior employing a modified Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. Devices that do not contain an HTL appear to degrade by a mechanism other than pinhole ingress resulting in a more uniform degradation of the photocurrent across the active area.     

Lifetimes of organic photovoltaics: Using TOF-SIMS and O2 isotopic labelling to characterise chemical degradation mechanisms

The lifetimes of organic photovoltaic cells based on conjugated polymer materials were studied. The device geometry was glass:ITO:PEDOT:PSS:C₁₂-PSV:C₆₀:aluminium. To characterise and elucidate the parts of the degradation mechanisms induced by molecular oxygen, ¹⁸O₂ isotopic labelling was employed in conjunction with time-of-flight secondary ion mass spectrometry. A comparison was made between devices being kept in the dark and devices that had been subjected to illumination under simulated sunlight (1000 W m⁻², AM1.5) and this demonstrated that oxygen-containing species were generated throughout the active layer with the largest concentration towards the aluminium electrode. For devices that had been kept in the dark oxygen species were only observed at the immediate interface between the aluminium and the organic layer. The isotopic labelling allowed us to demonstrate that the oxygen comes from the atmosphere and diffuses through the aluminium electrode and into the device.     

Characteristics of indium-free GZO/Ag/GZO and AZO/Ag/AZO multilayer electrode grown by dual target DC sputtering at room temperature for low-cost organic photovoltaics

We compared the electrical, optical, structural and surface properties of indium-free Ga-doped ZnO (GZO)/Ag/GZO and Al-doped ZnO (AZO)/Ag/AZO multilayer electrodes deposited by dual target direct current sputtering at room temperature for low-cost organic photovoltaics. It was shown that the electrical and optical properties of the GZO/Ag/GZO and AZO/Ag/AZO multilayer electrodes could be improved by the insertion of an Ag layer with optimized thickness between oxide layers, due to its very low resistivity and surface plasmon effect. In addition, the Auger electron spectroscopy depth profile results for the GZO/Ag/GZO and AZO/Ag/AZO multilayer electrodes showed no interfacial reaction between the Ag layer and GZO or AZO layer, due to the low preparation temperature and the stability of the Ag layer. Moreover, the bulk heterojunction organic solar cell fabricated on the multilayer electrodes exhibited higher power conversion efficiency than the organic solar cells fabricated on the single GZO or AZO layer, due to much lower sheet resistance of the multilayer electrode. This indicates that indium-free GZO/Ag/GZO and AZO/Ag/AZO multilayer electrodes are a promising low-cost and low-temperature processing electrode scheme for low-cost organic photovoltaics.     

Organic photovoltaic cells based on TPBi as a cathode buffer layer

The performance of organic photovoltaic (OPV) cells based on copper phthalocyanine (CuPc)/C₆₀ heterojunction was investigated by focusing on the role of 1,3,5-tris(2-N-phenylbenzimidazolyl) benzene (TPBi) as a cathode buffer layer. The effect of the film thickness of TPBi layer on the electrical characteristics of the device was systematically studied. The interface between the acceptor and cathode was studied with the characterization of atomic force microscope. Optical field distribution inside the OPV cell was also simulated to gain insight into the mechanism responsible for TPBi used as an optical spacer. The results indicated that at an optimal film thickness, TPBi cathode buffer layer is essential to enhance device performance by forming improved interfacial contact without introducing more series resistance and current loss.     

Making the case for grid-connected photovoltaics in Brazil

In the developed world, grid-connected photovoltaics (PVs) are the fastest-growing segment of the energy market. From 1999 to 2009, this industry had a 42% compound annual growth-rate. From 2009 to 2013, it is expected to grow to 45%, and in 2013 the achievement of grid parity – when the cost of solar electricity becomes competitive with conventional retail (including taxes and charges) grid-supplied electricity – is expected in many places worldwide. Grid-connected PV is usually perceived as an energy technology for developed countries, whereas isolated, stand-alone PV is considered as more suited for applications in developing nations, where so many individuals still lack access to electricity. This rationale is based on the still high costs of PV when compared with conventional electricity. We make the case for grid-connected PV generation in Brazil, showing that with the declining costs of PV and the rising prices of conventional electricity, urban populations in Brazil will also enjoy grid parity in the present decade. We argue that governments in developing nations should act promptly and establish the mandates and necessary conditions for their energy industry to accumulate experience in grid-connected PV, and make the most of this benign technology in the near future.     

Long-term scenarios for the integration of photovoltaics into the global energy system

The paper discusses the role of photovoltaics in current energy scenario work on a local, regional or global scale, for the mid-21st century.     

Lifetimes of organic photovoltaics: Combining chemical and physical characterisation techniques to study degradation mechanisms

Degradation mechanisms of a photovoltaic device with an Al/C₆₀/C₁₂-PSV/PEDOT:PSS/ITO/glass geometry was studied using a combination of in-plane physical and chemical analysis techniques: TOF-SIMS, AFM, SEM, interference microscopy and fluorescence microscopy. A comparison was made between a device being stored in darkness in air and a device that had been subjected to illumination under simulated sunlight (1000  W m^–2, AM1.5) in air. It was found that oxygen diffuses through pinholes in the aluminium electrode. If stored in air in the dark the oxidation is limited to the C₆₀ layer. Illumination accelerates the oxidation/degradation and thus expands the process to involve at least the underlying layer of C₁₂-PSV. Furthermore, it was found that particles are formed in the device during storage.     

有机太阳能电池应用前景展望

太阳能是可再生的巨大能源,充分利用太阳能是解决能源危机的重要途径之一。有机太阳能电池是实现将太阳能直接转变为电能的最有前景的器件之一。文章综述了有机太阳能电池的基本原理和应用,特别讨论了造成有机太阳电池性能低下的原因,并介绍了有机太阳电池的最新进展。     

Transparent anodes for polymer photovoltaics: Oxygen permeability of PEDOT

The oxygen permeability of the transparent organic anode poly(3,4,-ethylene dioxythiophene) with paratoluenesulphonate as the anion (PEDOT:pTS) was determined to be (STP) , and is thus comparable in magnitude to the oxygen permeability of polyethyleneterephthalate (PET). The oxygen diffusion through bilayers of polyethylene (PE) and PEDOT:pTS and bilayers of PET and PEDOT:pTS was established. The bilayer structures were applied as the carrier substrate and the transparent anode in polymer-based photovoltaic devices employing a mixture of poly(1-methoxy-4-(2-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV) and [6,6]-phenyl-C₆₁-butanoicacidmethylester (PCBM) as the active layer and aluminium as the cathode. The oxygen permeability of the layers and the aluminium cathode was correlated with the lifetime of the solar cell devices. It was found that the performance of the devices with PET as the carrier substrate degraded more slowly due to the lower oxygen and water permeability, whereas devices using PE as the carrier substrate gave devices with a very short lifetime. It was found that PEDOT:pTS on its own is a not a significant barrier for oxygen in the context of photovoltaic devices where long lifetimes are anticipated. It is concluded that the large oxygen permeability of the barrier layers contribute to the short device lifetimes while other permeates such as water also contribute to device degradation.     

Organic donor, acceptor and buffer layers of small molecules prepared by OVPD technique for photovoltaics

Organic vapour-phase deposition (OVPD^®) is used for the growth of the organic solar cell component materials such as the donor copper phthalocyanine (CuPc), the acceptor fullerene C₆₀, and electron-conducting buffer layers of bathocuproine (BCP) on Si1 0 0 wafers and indium tin oxide (ITO) substrates on areas as large as 15×15 cm². By means of X-ray diffraction (XRD) analysis we show that under continuous operating conditions the source materials possess long-term stability. The CuPc, C₆₀ and BCP thin film morphology and structure are characterised using scanning electron microscopy and XRD analysis. We demonstrate CuPc thin films with a highly folded surface morphology suitable for the preparation of solar cells with an interpenetrating donor–acceptor interface. The XRD diffraction patterns of the CuPc and C₆₀ layers deposited under conditions appropriate for the preparation of organic solar cells show spectra typical for these materials. Mixed CuPc:C₆₀ layers with controlled constituent ratios and layer thickness are deposited for the preparation of organic solar cells. First ITO/CuPc:C₆₀/Al organic photovoltaic devices are prepared with an efficiency of 1% (conditions AM1.5).     

Substrate to substrate aluminized bonding of 10 cm round silicon substrates for application in solar photovoltaics

Large area silicon solar cells with screen printed contacts have been realized for the first time on 10 cm diameter, p-type, Cz silicon wafers which were bonded to silicon substrates by alloying of a suitably thick screen printed layer of Al on them. In cells made on 300 μm thick wafers without texturization, antireflection coating and passivation of the front surface, the values of the open-circuit voltage (V_oc), the short-circuit current density (J_sc), curve factor (CF) and the efficiency (η) were found to be in the range 572–579 mV, 16–19.2 mA cm⁻², 0.53–0.61 and 5.5–5.89%, respectively, under simulated tungsten halogen light of 100 mW cm⁻² intensity. Using thinner wafers and having optical confinement, surface passivation and effective back surface field, the cell performance would be substantially improved. In fact, an efficiency close to 18% (AM1.5) would be realizable with this approach. Another attractive feature of this approach is that a low-cost silicon substrate could be used at the bottom that would act as support for the thin top surface without disadvantage to the cell performance. In this paper only the principle has been demonstrated experimentally. Possible improvements have been shown by computer simulation.     

Analysis of exhaust waste heat recovery from a dual fuel low temperature combustion engine using an Organic Rankine Cycle

This paper examines the exhaust waste heat recovery potential of a high-efficiency, low-emissions dual fuel low temperature combustion engine using an Organic Rankine Cycle (ORC). Potential improvements in fuel conversion efficiency (FCE) and specific emissions (NO_x and CO₂) with hot exhaust gas recirculation (EGR) and ORC turbocompounding were quantified over a range of injection timings and engine loads. With hot EGR and ORC turbocompounding, FCE improved by an average of 7 percentage points for all injection timings and loads while NO_x and CO₂ emissions recorded an 18 percent (average) decrease. From pinch-point analysis of the ORC evaporator, ORC heat exchanger effectiveness (?), percent EGR, and exhaust manifold pressure were identified as important design parameters. Higher pinch point temperature differences (PPTD) uniformly yielded greater exergy destruction in the ORC evaporator, irrespective of engine operating conditions. Increasing percent EGR yielded higher FCEs and stable engine operation but also increased exergy destruction in the ORC evaporator. It was observed that hot EGR can prevent water condensation in the ORC evaporator, thereby reducing corrosion potential in the exhaust piping. Higher ? values yielded lower PPTD and higher exergy efficiencies while lower ? values decreased post-evaporator exhaust temperatures below water condensation temperatures and reduced exergy efficiencies.